A better understanding of the cellular mechanisms required for the generation and maintenance of transplantation tolerance will require integrated applications of new technologies in cell cloning, monoclonal antibodies and protein purification to establish experimental transplantation models. We have developed and investigated heart and kidney transplantation models using cyclosporine in inbred rat strains. We have developed quantitative in vitro assays for classic helper, cytotoxic and suppressor T cell activities and applied these assays to the investigation of tolerance mechanisms. The results challenge the current paradigm that successful transplantation is measured by the simple presence of suppressor cells. Rather, tolerance must be considered in the context of a balance between helper/inducer and suppressor cell activities, between the generation of cytotoxic cells and antibodies that target and destroy the transplant and the suppressor cells that inhibit these mechanisms of graft rejection. Thus, we developed in vitro assays defining suppressor-inducer and suppressor precursor T cells. We have also identified a new suppressor-inducer lymphokine. These assays will permit the investigation of suppressor cell induction circuits operant in the immune response. The present proposal aims to build on this in vitro foundation by application to the in vivo study of experimental kidney transplantation. Thus, the immunoregulatory T cell profiles of cyclosporine treated transplanted animals will be assayed at sequential times following surgery. Our investigations will be extended by developing suppressor-inducer and suppressor precursor cell clones, monoclonal antibodies and a purified suppressor inducer lymphokine preparation. These new reagents will then be used to selectively manipulate the immune responses of transplanted animals in order to actively determine the roles of cyclosporine and suppressor induction circuits in transplantation tolerance.